1. Technical Field of the Invention
The present invention relates to an improvement of a rotation angle sensor that detects an angle of rotation of a rotary shaft by detecting the rotation of a magnetic field vector caused by the rotation of the rotary shaft.
2. Description of the Related Art
Steering angle sensors utilizing a rotation angle sensor are known. Such a rotation angle sensor detects the change in the angle of rotation of a magnet (including a polarized body), using a magnetic sensing element.
Japanese Patent Laid-Open Publication No. 2005-003625 and U.S. Pat. No. 6,894,487 each disclose this type of rotation angle sensor that uses a sensor that can detect an angle of rotation greater than 360 degrees (hereinafter also referred to as an “over-360 degrees rotation sensor”) of a rotary shaft whose angle of rotation is to be detected. The term “over-360 degree rotation sensor” refers to a sensor that detects the total number of degrees of revolution that the rotary shaft has undergone. For example, on the first revolution a turn of 45 degrees from the rotation start point will be measured as 45 degrees. On the next revolution, the same position will be measured as 360+45 degrees, i.e. 405 degrees.
U.S. Pat. No. 6,894,487 suggests an over-360 degrees rotation sensor having a structure in which a magnet is rotated while being concurrently moved in the axial direction. A magnetic sensor that is disposed axially close to the magnet determines the angle of rotation based on the direction of the magnetic flux density and determines the Nth rotation (where N represents the number of complete rotations that have occurred since rotation started) based on the intensity of the magnetic flux density.
In addition, Japanese Patent Laid-Open Publication No. 2007-256250 filed by the applicant of the present invention suggests an over-360 degrees rotation sensor having a structure in which a cone taper surface of a cylindrical magnet having an inner peripheral surface with a cone tapered surface (it is also simply called a coned surface) is magnetized so that the N pole and S pole are formed separately at 180 degrees mutually onto the cone taper surface. When the cone taper surface is rotated around a magnetic sensing element while being concurrently moved in the axial direction, the magnetic sensor determines the Nth rotation based on the intensity of the magnetic flux density.
In the over-360 degrees rotation sensor suggested in Japanese Patent Laid-Open Publication No. 2005-003625, two magnet shafts independently engage with a single rotary shaft whose angle of rotation is to be detected. The angles of rotations of the two magnet shafts are detected by two respective magnetic sensing elements.
The two magnetic sensing elements are adapted to generate outputs having different phase angles. A signal processing unit then calculates an angle of rotation over 360 degrees based on the difference between the phase angles of the two outputs.
The over-360 degrees rotation sensor of this literature can detect an angle of rotation over 360 degrees. However, this sensor is required to arrange two sets of gear mechanisms, magnets and magnetic sensing elements around the rotary shaft subjected to detection.
Thus, the sensor disclosed in this literature has suffered from such problems as the increases in the number of parts and the size of the sensor, as well as the increase in the manufacturing cost.
A single-axis over-360 degrees rotation sensor explained below can mitigate these problems of the two-axis over-360 degrees rotation sensor.
In the over-360 degrees rotation sensor suggested in U.S. Pat. No. 6,894,487, there was a problem that separating the magnetic flux affecting the magnetic sensing element and an external noise in the magnetic field is not easy, and signal to noise ratio (S/N ratio) is poor.
In the over-360 degrees rotation sensor suggested in Japanese Patent Laid-Open Publication No. 2007-256250, although the circumference of the magnetic sensing element is surrounded by the magnet or the yoke, the influence of the external noise in the magnetic field may be reduced and the detection accuracy may be improved, the manufacturing and magnetization processes become complicated because the inner peripheral surface of the magnet is the cone tapered surface, and many magnet material may be consumed.
In addition, there is a problem that it is easy to cause a crack in the magnet because the thickness in the direction of the diameter of each part of the magnet is different.